text: Aspen HYSYS (or simply HYSYS) is a chemical process simulator currently developed by AspenTech used to mathematically model chemical processes, from unit operations to full chemical plants and refineries. HYSYS is able to perform many of the core calculations of chemical engineering, including those concerned with mass balance, energy balance, vapor-liquid equilibrium, heat transfer, mass transfer, chemical kinetics, fractionation, and pressure drop.   HYSYS is used extensively in industry and academia for steady-state and dynamic simulation, process design, performance modeling, and optimization.